The present invention relates to improved emulsions and methods of using such emulsions. More particularly, the present invention relates to surfactant-free emulsions and their use in subterranean applications.
Emulsions usually comprise two immiscible phases. The two immiscible phases include a continuous (or external) phase and a discontinuous (or internal) phase. The discontinuous phase comprises the secondary fluid that usually exists in droplets in the continuous phase. Two varieties of emulsions are oil-in-water and water-in-oil. Oil-in-water emulsions usually include a fluid at least partially immiscible in an oleaginous fluid (usually an aqueous-based fluid) as the continuous phase and an oleaginous fluid as the discontinuous phase. Water-in-oil emulsions are the opposite, having the oleaginous fluid as the continuous phase and a fluid at least partially immiscible in the oleaginous fluid (usually an aqueous-based fluid) as the discontinuous phase. Water-in-oil emulsions may be also referred to as invert emulsions.
Such emulsions have been used widely in oil and gas applications. For instance, emulsions may be used in the oil and gas industry for subterranean treatment applications, including drilling, production, and completion operations. Invert emulsions may be used because oleaginous-based treatment fluids (also known as muds) may have superior performance characteristics when compared with water-based muds in some situations, e.g., when there is an abundance of water reactive materials in a well bore. These superior performance characteristics may include, e.g., better lubrication of the drilling strings and downhole tools, thinner filter cake formation, and better hole stability. An emulsification of water-in-oil, without having any emulsifying agent capable of stabilizing the fluid that is at least partially immiscible in the oleaginous fluid typically will undergo natural degradation processes including droplet coalescence and Ostwald ripening, until the two phases which are at least partially immiscible separate and the emulsion no longer exists. Having an unstable invert emulsion may be problematic because if the emulsion destabilizes, it may not have consistent, reliable properties. This problem may be exacerbated by the physical forces that the emulsion may undergo when being used in subterranean applications, such as thermal, mechanical, and chemical stresses. Emulsion stabilizing agents, sometimes referred to as emulsifiers, may be useful in invert emulsions to stabilize the emulsions, especially when used in subterranean applications.
Some traditional emulsion stabilizing agents are surfactant-based. Surfactant-based emulsion stabilizing agents usually comprise a hydrophobic part that is attracted to the oil phase and a hydrophilic part that is attracted to the water phase. Generally, the hydrophobic part interacts with the oil and the hydrophilic part interacts with the nonoleaginous fluid. These interactions generally decrease the surface tension of the interface between the water droplet and the oil, which may slow the natural tendency of the two immiscible phases to separate.
However, surfactant-based emulsion stabilizing agents may be problematic, as they may suffer from problems that include toxicity, limited range of oil to water ratios, thermal destabilization propensity for droplet coalescence, and intolerance to various salts and other chemical agents. The toxicity of the surfactants can create potential dangers for those using them and the environment. Further, a surfactant is typically capable of stabilizing either an oil-in-water emulsion or a water-in-oil emulsion, but not both. Because of the necessity to carefully balance the chemical interactions of the surfactant to the type of micelle formed, typical surfactants generally can be used only with a limited oil to water ratio range. Surfactants also may have adverse effects on shrimp and other aqueous species, along with poor biodegradability. Traditional surfactant-based emulsion agents also may have a propensity for droplet coalescence, which may be undesirable. Surfactant-based agents typically form a meta-stable structure around the micelle because they diffuse in and out of the micelles. This meta-stable structure promotes such forces as coalescence, which may result in phase separation and eventual emulsion instability. Lastly, traditional surfactant-based stabilizing agents may be intolerant to salts and various other chemical agents.